The invention relates to a device for changing a length of a running path of an electromagnetic wave, according to the features specified in the preamble of claim 1.
Prior art electromagnetic wave devices were serve for changing the time of travel of a beam, for producing a Doppler shift, for producing a time-dependent spectrum or likewise. They are for example used with optical coherence tomography (OCT), with absorption measurement or also with Fourier transform spectroscopy. Generally such devices can be applied when it is necessary to change the optical path length of a beam path.
From U.S. Pat. No. 5,220,463 there is known such a device. Here the change of a path length of the light beam is produced by way of a crank drive, wherein at the end of the crank rod there is arranged a retroreflector which runs on a parallel guide so that the reflector or the reflector pair with each rotation of the crank moves forwards and backwards by a path length which corresponds to double the distance between the linkage point of the crank rod and the rotational axis of the shaft. The light beam runs roughly five times through the space lying between the reflection means before it leaves the arrangement deflected by a mirror transverse to the incidence direction. The change in wavelength and change in time of travel to be achieved by way of this is comparatively low since the light beam is thrown to and fro only a few times between the reflection means before it exits. An enlargement of the mirror reflectors or retroreflectors may here hardly provide any help since with an increasing size also the inertia properties of the moved parts are negatively influenced. The system thus meets its physical limits. A further disadvantage is that on account of the conversion of the rotary drive into a translatory movement the translatory movement is not linear but dependent on the angle position of the crank.
Inasmuch as this is concerned the device of U.S. Pat. No. 5,784,186 is more favorable with which the mirror arrangements are arranged on a rotating disk. With this although indeed a practical path length change is effected over time, however this path length change itself is quite small since the beam is only reflected once and thus a multiplication factor is absent.
The problems resulting in this context in particular with coherence tomography are for example described in the article xe2x80x9cin vivo video rate optical coherence tomographyxe2x80x9d of Andrew M. Rollins amongst others in OPTIC EXPRESS Vol. 3, No. 6 of 14.09.1998, pages 219 to 229, in particular on page 221.
Against this state of the art it is the object of the invention to design a device for changing the path length of the running path of an electromagnetic wave, in particular a light beam such that as large, quick and controlled as possible variation of the time of travel is possible with simple means.
This object is achieved according to the invention by the features specified in the following description and claims. Advantageous formations of the invention are specified in the dependent claims as well as the subsequent description.
In one aspect, the invention comprises a device for changing a length of a running path of an electromagnetic wave, in particular of a light beam, with two reflection means arranged lying opposite, between which the beam is reflected, as well as with a means for changing a distance (a) of the reflection means, wherein the reflection means enclose an angle xcex2, wherein 0xc2x0 less than xcex2 less than 45xc2x0.
In another embodiment, the invention comprises a method for changing a length of an electromagnetic wave comprising the steps of situating a first reflector a distance (a) and in opposed relation to a second reflector; the first and second reflectors enclosing an angle xcex2, wherein 0xc2x0 less than xcex2 less than 45xc2x0; and reflecting the light between said first and second reflectors.
In another aspect, this invention comprises a device for changing a length of a running path of an electromagnetic wave, in particular of a light beam, with a first and second reflector arrange lying opposite, between which the beam is reflected, as well as with a driver for changing a distance (a) of the first and second reflectors, wherein said first and second reflectors enclose an angle xcex2, wherein 0xc2x0 less than xcex245xc2x0.
According to the invention there are provided at least two oppositely lying reflection means between which the beam is reflected and whose distance may be changed. The arrangement of the reflection means is effected with this at an angle xcex2 to one another which lies between 0xc2x0 and 45xc2x0 (0xc2x0 less than xcex2 less than 45xc2x0). The reflection means are thus arranged to one another such that they span an acute angle in a manner such that the light beam is thrown to an fro several times between the reflection means and with this runs through the whole width of the reflection means until finally the light beam at an angle of 90xc2x0 impinges one of the reflection means and then runs back the same way. By way of this arrangement on the one hand a large path length change is achieved, since with a suitable selection of the angle the light beam is thrown to and fro many times until it has run through the width of the reflection means, and on the other hand there does not exist the danger that by way of distance change of the reflection means the light beam xe2x80x9cfalls out of the systemxe2x80x9d. A further advantage of this arrangement is that the incident and the emergent beam coincide with a suitable arrangement. By way of the multitude of reflections between the means, already with a small displacement of the reflection means to one another a comparatively large path length change is achieved.
For example a mirror, a prism, a retroreflector or likewise may serve as a reflection means. In the simplest form the reflection means are formed by two mirrors arranged at an acute angle to one another, wherein the acute angle xcex2 lies preferably in the region between 0.01xc2x0 and 5xc2x0. In order to let the beam reflect as often as possible between the mirrors, the angle xcex2 is selected as flat as possible, thus the mirrors are arranged almost parallel to one another.
Instead of a mirror pair also a mirror may be combined with an oppositely lying retroreflector. The path length may be doubled by using two prisms as oppositely lying reflection means, wherein then when the incident and emergent beam are to coincide additionally a third reflection means is to be provided preferably in the form of a mirror.
The change of the distance of the reflection means to one another may be effected in a manner known per se, for example electromotorically by way of an eccentric drive, by way of an electromagnetic drive (e.g. moving coil), by way of a pneumatic drive or likewise. Preferably the drive is effected piezoelectrically, and specifically via a stack of piezoelements. The stack arrangement, i.e. the arrangement of several piezoelements behind one another ensures a sufficiently large displacement and thus the desired path length change. Alternatively to the stack arrangement also a single piezoelement may be provided which is impinged with a correspondingly high voltage in order to achieve the desired displacement. Since the piezoelements react very quickly with this also the control of a speed and path profile is possible which opens up further fields of application of the device. If additionally a distance transducer may be attached to the device such that the actual deflection of the movable deflection means may be determined there arises the possibility of controlling the position or speed of the reflection means to a predetermined profile. Selectively one or both of the oppositely lying reflection means may be arranged movable so that the path change with a suitable activation of the drive elements may be again increased.
In order to ensure that with the distance change the angle xcex2 of the reflection means to one another remains constant it is useful to suitably guide the movable reflections means. The parallel guiding may be effected in a manner known per se, for example by way of a sliding bearing, by way of a pneumatic bearing or likewise. Preferably the guiding of the movable part or of the movable parts is effected by way of a solid body joint, in particular a parallel leaf spring arrangement.
According to the used reflection means differing demands are to be fulfilled by the guide. If the movable reflection means is a mirror the guide firstly is to permit of course the desired translation along the Z-axis (see FIG. 2). If in the context with the desired translation an additional translatory movement along the Y-axis (see FIG. 2) sets in, this does not limit the manner of functioning of the device. The guide must however effectively suppress parasitic rotation movements about the Y and X axes (see FIG. 2) (yawing and pitching) (smaller than a few irad in the preferred embodiment example), since otherwise it may not be ensured that the beam exiting the device remains congruent with the beam entering the device.
If the movable reflection means is a prism the demands on a suitable tilting about the X-axis may be considerably lower since the use of a prism renders the device invariant against pitching movements. With the use of a retroreflector as a movable reflection means the device becomes invariant against all rotatory movements.
The field of application of the device according to the invention is varied. A preferred application is the use as a multipath cell in spectroscopy. With this the gas to be examined is located between the two reflection means. A further advantageous application of the device as a multipath cell lies in Fourier transform spectroscopy, wherein by way of the movable reflection means the necessary length change of the sample path may be achieved. Also the multipath cell formed with the device according to the invention may be used for absorption measurement, wherein by way of the movable reflection means the length of the absorption path is changed so that a differential measurement of the absorption coefficients is possible. A preferred application of the device lies in the optical coherence tomography, wherein the device here serves as a reference path of the used interferometer.